Rocks are a form of matter. All the matter you see, including rocks, is made of tiny particles called atoms. When all the atoms of a particular type of matter are the same, the matter is an element.
Most elements are stable . They do not change under normal conditions. But some elements exist in forms that are unstable . Over time, these elements break down, or decay, by releasing particles and energy in a process called radioactive decay . These unstable elements are said to be radioactive .
During radioactive decay, the atoms of one element break down to form atoms of another element . Radioactive elements occur naturally in igneous rocks.(rocks formed from magma). For an igneous rock, its "birthday" is when it first hardens to become rock. As a radioactive element within the igneous rock decays, it changes into another element. Therefore, the composition of the rock changes slowly over time. The amount of the radioactive element (parent) goes down. But the amount of the new element (daughter) goes up. The rate of decay of each radioactive element is constant; it never changes. This rate of decay is the element's half-life . The half-life of a radioactive element is the time it takes for half of the radioactive atoms (parent) to decay, or change into a stable element(daughter). Scientists use this rate of decay to calculate a rock’s age.
Geologists use radioactive dating to determine the absolute age of rocks.
The absolute age of a rock is its age in numbers of years . In radioactive dating, scientists first determine the amount of a radioactive element in a rock. Then they compare that amount with the amount of the stable element into which the radioactive element decays. So if a rock has 50% parent element X and 50% daughter element Z, and the half life of the
X is 100 years, the rock would be 100 years old. If it was 25 % X and 75% Z that means it went through 2 half lives and is 200 years old.

Scientists often date rocks using potassium-40. This form of potassium decays to form the stable element argon-40 and has a half-life of 1.3 billion years. The long half-life of potassium-40 makes it useful in dating the most ancient rocks.
All plants and animals contain some carbon-14, a radioactive form of carbon which changes to Nitrogen 14. Carbon-14 is useful in dating materials from organic substances-plants and animals- that lived less than 50,000 years ago. Because carbon-14 has a half-life of only 5,730 years, it can't be used to date more ancient fossils or rocks. You cannot use radiocarbon dating to determine the age of rocks because they were never alive. Radioactive dating works well for dating igneous rocks but not for sedimentary rocks. Rock particles in sedimentary rocks are from other rocks, all of different ages. Radioactive dating would provide the ages of particles, not the sedimentary rock as a whole. But radioactive dating can be used to determine absolute dates of igneous intrusions and extrusions n ear sedimentary rock layers.
Igneous intrusions are pockets of igneous rock that formed when magma pushed up through cracks and hardened into rock forming dikes, sills, baccoliths, and other formations. Intrusions are always younger than the layers of rock they push through. Igneous extrusions are rocks that formed from lava on the surface of the earth.
Sedimentary rock above an igneous intrusion must be younger than that intrusion.
Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B.
Sedimentary Rock D is younger than Erosion Surface C.
Diagram (2) Sill B is younger than
Sedimentary Rock A. Dike C is younger than sill B.
Diagram (3) Stock B is younger than
Sedimentary Rock A. Dike C is the youngest.

Name _________________________________
1. A type of matter in which all the atoms making up the matter are the same is called a (n) ______________.
2. What does the word stable in paragraph 2 mean? _______________________________________
_________________________________________________________________________________
3. What are unstable elements called? __________________________________________________
4. What occurs during radioactive decay? ________________________________________________
_________________________________________________________________________________
5. Circle the letter of the kind of rock for which radioactive dating works well. a. sedimentary b. igneous c. metamorphic d. layered
6. Carbon -14 changes to Nitrogen 14. Which is the parent element and which is the daughter element? Parent__________________ and Daughter_______________________
7. What is the half life of an element?_________________________________________________
_______________________________________________________________________________
8. What is another word for decay? ________________________ Why is it used in the word radioactive decay? _______________________________________________________________
9. True or False? Scientists use the rate at which radioactive elements decay in rocks to determine its
relative age.
10. Circle the letter of each sentence that is true about radioactive decay. a. Over time, the amount of a radioactive element in igneous rock will go up. b. The rate of decay of each radioactive element is always changing. c. The rate of radioactive decay is an element's half-life. d. The half-life of a radioactive element is the time it takes for half of all the atoms in a rock to decay.
11. True or False? Geologists use radioactive dating to find the absolute age of rocks.
12. What is meant by the absolute age of a rock? _________________________________________
13. What two things must scientists measure to find the absolute age of a rock?
a.. ___________________________________________________________________________________________________ b. ____________________________________________________________

14. True or False? By comparing the amount of the radioactive element with the amount of the stable element, scientists can determine the absolute age of a rock if they know how long it takes for the radioactive element to become stable.
15. Complete the table to compare two different types of radioactive dating.
Elements Used In Radioactive Dating
Elements
Potassium-40
Decays to
A.
Used for Dating
B.
Half-life
C.
Carbon-14 D. E. F.
16. Suppose a geologist wants to determine the absolute age of an igneous rock. The geologist thinks the rock might have formed about 400 million years ago. Would you use potassium-40 or carbon-14 to date the igneous rock? Explain your answer.
___________________________________________________________________________
___________________________________________________________________________
17. True or False? Carbon -14 can only be used to date organic materials less than 50,000 years old
.
18. True or False? Radioactive dating can only be used for igneous rocks, not sedimentary rocks.
19. Explain your answer for number 13._______________________________________________
_______________________________________________________________________________
20. What do scientists use to help them date sedimentary layers?
_____________________________________________________________________________
21. True or False? Igneous intrusions are always older than the sedimentary layers they cut through.
22. Intrusions are made of _______________ while extrusions are made of _______________.
23. A horizontal intrusion is called a _____________. A vertical intrusion is called a ______________ and.
a large igneous intrusion covering several layers of rock in all directions is called a ____________________.
( Use the last diagram on the reading sheet to find the answer )